(1) Field of the Invention
This invention relates to a process for producing recording papers such as ink jet recording paper, optical bar code printing paper and the like.
(2) Description of the Prior Art
Having various characteristic features such as high-speed printability, low noisiness, great versatility of recorded pattern, easiness of multi-color printing and so on, ink jet recording process has held an important position in the recent years in various fields including information instruments. Further, the image formed by the multi-color ink jet process is by no means inferior to that formed by the usual multi-color printing process. In addition, multi-color ink jet process necessitates no use of printing plate and therefore is less expensive than multi-color printing by the usual process using printing plate, so far as the number of prints to be made is small. For these reasons, it is being attempted currently to expand the application of ink jet process even to the field of multi-color printing instead of limiting its application only to the field of recording.
Since art paper and coated paper used in the usual printing processes are very poor in ink-absorbability, the ink remains for a long period of time on their surface after completion of ink jet recording, which can cause damage on the image when the operator touches some part of the apparatus or the recorded surface is rubbed. Further, in the part where the image has a high color density, the large amounts of inks can mix one another before they are absorbed or can flow out of the place. Therefore, the use of these papers in ink jet process is impractical.
A recording sheet which can be used in ink jet process has to satisfy the following requirements simultaneously. Thus, it must give a clear image of high color density; it must rapidly absorb the ink enough to prevent the flow-out of ink; and in addition it must suppress the diffusion of ink dot towards the horizontal direction on its surface in order to enhance the resolution.
However, as is self-evident, there is such a relation between the ink-absorbability and the ink diffusion characteristics to the horizontal direction relating to resolution that an enhancement of absorbability results in an increase in diffusion to horizontal direction and a control of diffusion to horizontal direction results in an decrease in absorbability. With the aim of solving these problems, control of the sizing property of paper or incorporation of filler having a great specific surface area such as clay, talc, calcium carbonate, urea-formaldehyde resin or the like at the time of paper-making is practised actually, and products having a certain extent of adaptability to ink jet are manufactured by these techniques. However, most of these products cannot give an image having a clear color tone and cannot give an image so attractive in appearance as that obtained by usual multi-color printing such as offset printing process, even though they may partially fulfil the above-mentioned adaptabilities to ink jet. For example, an ink jet recording paper coated with a pigment of high ink-absorbability such as non-colloidal silica powder is disclosed in Japanese Patent Application Kokai (Laid-Open) No. 51,583/80, and an optical bar code printing paper coated with finely powdered silica is disclosed in Japanese Patent Publication No. 790/78. The silica powders used in these techniques require to use a large amount of binder to bond them. For example, as is mentioned in Japanese Patent Application Kokai (Laid-Open) No. 51,583/80, so large an amount as 20-150 parts of binder must be used per 100 parts of silica. An increased amount of binder causes the occurrence of many minute cracks in the dried coating layer, which decrease the resolution because ink runs along the cracks.
On the other hand, in the optical bar code printing paper of Japanese Patent Publication No. 790/78, 5-20 parts of binder is used per 100 parts of silica. Generally, resolution can be improved by increasing the amount of silica coated, and accordingly the amount of silica coated must be 10 g/m.sup.2 or more in order to obtain a sufficient resolution. However, when the binder is used only in an amount of 5-20 parts, the silica can readily peel off from the paper layer so that a coating layer giving a sufficient resolution cannot be obtained. That is to say, there is a tendency that the resolution, important to the adaptability to ink jet, decreases whether the proportion of binder is increased or the amount of coating is decreased.